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diff --git a/dom/svg/SVGLengthList.h b/dom/svg/SVGLengthList.h
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+++ b/dom/svg/SVGLengthList.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef DOM_SVG_SVGLENGTHLIST_H_
+#define DOM_SVG_SVGLENGTHLIST_H_
+
+#include "nsCOMPtr.h"
+#include "nsDebug.h"
+#include "nsIContent.h"
+#include "nsINode.h"
+#include "nsIWeakReferenceUtils.h"
+#include "SVGElement.h"
+#include "nsTArray.h"
+#include "SVGLength.h"
+#include "mozilla/dom/SVGLengthBinding.h"
+
+namespace mozilla {
+
+namespace dom {
+class DOMSVGLength;
+class DOMSVGLengthList;
+}  // namespace dom
+
+/**
+ * ATTENTION! WARNING! WATCH OUT!!
+ *
+ * Consumers that modify objects of this type absolutely MUST keep the DOM
+ * wrappers for those lists (if any) in sync!! That's why this class is so
+ * locked down.
+ *
+ * The DOM wrapper class for this class is DOMSVGLengthList.
+ */
+class SVGLengthList {
+  friend class dom::DOMSVGLength;
+  friend class dom::DOMSVGLengthList;
+  friend class SVGAnimatedLengthList;
+
+ public:
+  SVGLengthList() = default;
+  ~SVGLengthList() = default;
+
+  // Only methods that don't make/permit modification to this list are public.
+  // Only our friend classes can access methods that may change us.
+
+  /// This may return an incomplete string on OOM, but that's acceptable.
+  void GetValueAsString(nsAString& aValue) const;
+
+  bool IsEmpty() const { return mLengths.IsEmpty(); }
+
+  uint32_t Length() const { return mLengths.Length(); }
+
+  const SVGLength& operator[](uint32_t aIndex) const {
+    return mLengths[aIndex];
+  }
+
+  bool operator==(const SVGLengthList& rhs) const;
+
+  bool SetCapacity(uint32_t size) {
+    return mLengths.SetCapacity(size, fallible);
+  }
+
+  void Compact() { mLengths.Compact(); }
+
+  // Access to methods that can modify objects of this type is deliberately
+  // limited. This is to reduce the chances of someone modifying objects of
+  // this type without taking the necessary steps to keep DOM wrappers in sync.
+  // If you need wider access to these methods, consider adding a method to
+  // SVGAnimatedLengthList and having that class act as an intermediary so it
+  // can take care of keeping DOM wrappers in sync.
+
+ protected:
+  /**
+   * This may fail on OOM if the internal capacity needs to be increased, in
+   * which case the list will be left unmodified.
+   */
+  nsresult CopyFrom(const SVGLengthList& rhs);
+
+  SVGLength& operator[](uint32_t aIndex) { return mLengths[aIndex]; }
+
+  /**
+   * This may fail (return false) on OOM if the internal capacity is being
+   * increased, in which case the list will be left unmodified.
+   */
+  bool SetLength(uint32_t aNumberOfItems) {
+    return mLengths.SetLength(aNumberOfItems, fallible);
+  }
+
+ private:
+  // Marking the following private only serves to show which methods are only
+  // used by our friend classes (as opposed to our subclasses) - it doesn't
+  // really provide additional safety.
+
+  nsresult SetValueFromString(const nsAString& aValue);
+
+  void Clear() { mLengths.Clear(); }
+
+  bool InsertItem(uint32_t aIndex, const SVGLength& aLength) {
+    if (aIndex >= mLengths.Length()) aIndex = mLengths.Length();
+    return !!mLengths.InsertElementAt(aIndex, aLength, fallible);
+  }
+
+  void ReplaceItem(uint32_t aIndex, const SVGLength& aLength) {
+    MOZ_ASSERT(aIndex < mLengths.Length(),
+               "DOM wrapper caller should have raised INDEX_SIZE_ERR");
+    mLengths[aIndex] = aLength;
+  }
+
+  void RemoveItem(uint32_t aIndex) {
+    MOZ_ASSERT(aIndex < mLengths.Length(),
+               "DOM wrapper caller should have raised INDEX_SIZE_ERR");
+    mLengths.RemoveElementAt(aIndex);
+  }
+
+  bool AppendItem(SVGLength aLength) {
+    return !!mLengths.AppendElement(aLength, fallible);
+  }
+
+ protected:
+  /* Rationale for using nsTArray<SVGLength> and not nsTArray<SVGLength, 1>:
+   *
+   * It might seem like we should use AutoTArray<SVGLength, 1> instead of
+   * nsTArray<SVGLength>. That would preallocate space for one SVGLength and
+   * avoid an extra memory allocation call in the common case of the 'x'
+   * and 'y' attributes each containing a single length (and the 'dx' and 'dy'
+   * attributes being empty). However, consider this:
+   *
+   * An empty nsTArray uses sizeof(Header*). An AutoTArray<class E,
+   * uint32_t N> on the other hand uses sizeof(Header*) +
+   * (2 * sizeof(uint32_t)) + (N * sizeof(E)), which for one SVGLength is
+   * sizeof(Header*) + 16 bytes.
+   *
+   * Now consider that for text elements we have four length list attributes
+   * (x, y, dx, dy), each of which can have a baseVal and an animVal list. If
+   * we were to go the AutoTArray<SVGLength, 1> route for each of these, we'd
+   * end up using at least 160 bytes for these four attributes alone, even
+   * though we only need storage for two SVGLengths (16 bytes) in the common
+   * case!!
+   *
+   * A compromise might be to template SVGLengthList to allow
+   * SVGAnimatedLengthList to preallocate space for an SVGLength for the
+   * baseVal lists only, and that would cut the space used by the four
+   * attributes to 96 bytes. Taking that even further and templating
+   * SVGAnimatedLengthList too in order to only use nsTArray for 'dx' and 'dy'
+   * would reduce the storage further to 64 bytes. Having different types makes
+   * things more complicated for code that needs to look at the lists though.
+   * In fact it also makes things more complicated when it comes to storing the
+   * lists.
+   *
+   * It may be worth considering using nsAttrValue for length lists instead of
+   * storing them directly on the element.
+   */
+  FallibleTArray<SVGLength> mLengths;
+};
+
+/**
+ * This SVGLengthList subclass is for SVGLengthListSMILType which needs to know
+ * which element and attribute a length list belongs to so that it can convert
+ * between unit types if necessary.
+ */
+class SVGLengthListAndInfo : public SVGLengthList {
+ public:
+  SVGLengthListAndInfo()
+      : mElement(nullptr), mAxis(0), mCanZeroPadList(false) {}
+
+  SVGLengthListAndInfo(dom::SVGElement* aElement, uint8_t aAxis,
+                       bool aCanZeroPadList)
+      : mElement(do_GetWeakReference(static_cast<nsINode*>(aElement))),
+        mAxis(aAxis),
+        mCanZeroPadList(aCanZeroPadList) {}
+
+  void SetInfo(dom::SVGElement* aElement, uint8_t aAxis, bool aCanZeroPadList) {
+    mElement = do_GetWeakReference(static_cast<nsINode*>(aElement));
+    mAxis = aAxis;
+    mCanZeroPadList = aCanZeroPadList;
+  }
+
+  dom::SVGElement* Element() const {
+    nsCOMPtr<nsIContent> e = do_QueryReferent(mElement);
+    return static_cast<dom::SVGElement*>(e.get());
+  }
+
+  /**
+   * Returns true if this object is an "identity" value, from the perspective
+   * of SMIL. In other words, returns true until the initial value set up in
+   * SVGLengthListSMILType::Init() has been changed with a SetInfo() call.
+   */
+  bool IsIdentity() const {
+    if (!mElement) {
+      MOZ_ASSERT(IsEmpty(), "target element propagation failure");
+      return true;
+    }
+    return false;
+  }
+
+  uint8_t Axis() const {
+    MOZ_ASSERT(mElement, "Axis() isn't valid");
+    return mAxis;
+  }
+
+  /**
+   * The value returned by this function depends on which attribute this object
+   * is for. If appending a list of zeros to the attribute's list would have no
+   * effect on rendering (e.g. the attributes 'dx' and 'dy' on <text>), then
+   * this method will return true. If appending a list of zeros to the
+   * attribute's list could *change* rendering (e.g. the attributes 'x' and 'y'
+   * on <text>), then this method will return false.
+   *
+   * The reason that this method exists is because the SMIL code needs to know
+   * what to do when it's asked to animate between lists of different length.
+   * If this method returns true, then it can zero pad the short list before
+   * carrying out its operations. However, in the case of the 'x' and 'y'
+   * attributes on <text>, zero would mean "zero in the current coordinate
+   * system", whereas we would want to pad shorter lists with the coordinates
+   * at which glyphs would otherwise lie, which is almost certainly not zero!
+   * Animating from/to zeros in this case would produce terrible results.
+   *
+   * Currently SVGLengthListSMILType simply disallows (drops) animation between
+   * lists of different length if it can't zero pad a list. This is to avoid
+   * having some authors create content that depends on undesirable behaviour
+   * (which would make it difficult for us to fix the behavior in future). At
+   * some point it would be nice to implement a callback to allow this code to
+   * determine padding values for lists that can't be zero padded. See
+   * https://bugzilla.mozilla.org/show_bug.cgi?id=573431
+   */
+  bool CanZeroPadList() const {
+    // NS_ASSERTION(mElement, "CanZeroPadList() isn't valid");
+    return mCanZeroPadList;
+  }
+
+  // For the SMIL code. See comment in SVGLengthListSMILType::Add().
+  void SetCanZeroPadList(bool aCanZeroPadList) {
+    mCanZeroPadList = aCanZeroPadList;
+  }
+
+  nsresult CopyFrom(const SVGLengthListAndInfo& rhs) {
+    mElement = rhs.mElement;
+    mAxis = rhs.mAxis;
+    mCanZeroPadList = rhs.mCanZeroPadList;
+    return SVGLengthList::CopyFrom(rhs);
+  }
+
+  // Instances of this special subclass do not have DOM wrappers that we need
+  // to worry about keeping in sync, so it's safe to expose any hidden base
+  // class methods required by the SMIL code, as we do below.
+
+  /**
+   * Exposed so that SVGLengthList baseVals can be copied to
+   * SVGLengthListAndInfo objects. Note that callers should also call
+   * SetInfo() when using this method!
+   */
+  nsresult CopyFrom(const SVGLengthList& rhs) {
+    return SVGLengthList::CopyFrom(rhs);
+  }
+  const SVGLength& operator[](uint32_t aIndex) const {
+    return SVGLengthList::operator[](aIndex);
+  }
+  SVGLength& operator[](uint32_t aIndex) {
+    return SVGLengthList::operator[](aIndex);
+  }
+  bool SetLength(uint32_t aNumberOfItems) {
+    return SVGLengthList::SetLength(aNumberOfItems);
+  }
+
+ private:
+  // We must keep a weak reference to our element because we may belong to a
+  // cached baseVal SMILValue. See the comments starting at:
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=515116#c15
+  // See also https://bugzilla.mozilla.org/show_bug.cgi?id=653497
+  nsWeakPtr mElement;
+  uint8_t mAxis;
+  bool mCanZeroPadList;
+};
+
+/**
+ * This class wraps SVGLengthList objects to allow frame consumers to process
+ * SVGLengthList objects as if they were simply a list of float values in user
+ * units. When consumers request the value at a given index, this class
+ * dynamically converts the corresponding SVGLength from its actual unit and
+ * returns its value in user units.
+ *
+ * Consumers should check that the user unit values returned are finite. Even
+ * if the consumer can guarantee the list's element has a valid viewport
+ * ancestor to resolve percentage units against, and a valid presContext and
+ * ComputedStyle to resolve absolute and em/ex units against, unit conversions
+ * could still overflow. In that case the value returned will be
+ * numeric_limits<float>::quiet_NaN().
+ */
+class MOZ_STACK_CLASS SVGUserUnitList {
+ public:
+  SVGUserUnitList() : mList(nullptr), mElement(nullptr), mAxis(0) {}
+
+  void Init(const SVGLengthList* aList, dom::SVGElement* aElement,
+            uint8_t aAxis) {
+    mList = aList;
+    mElement = aElement;
+    mAxis = aAxis;
+  }
+
+  void Clear() { mList = nullptr; }
+
+  bool IsEmpty() const { return !mList || mList->IsEmpty(); }
+
+  uint32_t Length() const { return mList ? mList->Length() : 0; }
+
+  /// This may return a non-finite value
+  float operator[](uint32_t aIndex) const {
+    return (*mList)[aIndex].GetValueInUserUnits(mElement, mAxis);
+  }
+
+  bool HasPercentageValueAt(uint32_t aIndex) const {
+    const SVGLength& length = (*mList)[aIndex];
+    return length.GetUnit() ==
+           dom::SVGLength_Binding::SVG_LENGTHTYPE_PERCENTAGE;
+  }
+
+ private:
+  const SVGLengthList* mList;
+  dom::SVGElement* mElement;
+  uint8_t mAxis;
+};
+
+}  // namespace mozilla
+
+#endif  // DOM_SVG_SVGLENGTHLIST_H_